BALTIMORE, AUG. 18 -- The Baltimore Orioles and No. 1 draft choice Ben McDonald concluded nine weeks of often bitter negotiations today by reaching agreement on a three-year contract worth $950,000, sources said. McDonald and his family will fly to Baltimore Saturday morning for an afternoon press conference at Memorial Stadium. The contract would be worth more than $1.1 million if McDonald -- the highest rated pitching prospect in history -- reaches all the available incentives, including rookie of the year and the Cy Young and most valuable player awards. The $950,000 is the second-largest contract ever given an amateur baseball player. Only Bo Jackson received more, and his case was unique because he was lured from the National Football League's Tampa Bay Buccaneers with a $1.1 million offer. Last year's No. 1 draft pick, Andy Benes, received $240,000 from the San Diego Padres. McDonald will make his professional debut early next week -- perhaps as early as Tuesday -- for a team in the Baltimore farm system. It's likely he'll be promoted to the majors in September, although that isn't guaranteed. The Orioles want to see McDonald pitch before making that decision. He has been working out three times a week in Louisiana, but has pitched competitively only once since the first week of June. It's extremely unlikely he would be added to the 24-man playoff roster when it's set Aug. 31. McDonald's agent, Scott Boras, and Orioles President Lawrence Lucchino reached the agreement after two negotiating sessions that covered about 13 hours. Boras phoned Lucchino Tuesday night and asked that negotiations be reopened. Boras flew from Los Angeles to Washington Wednesday night and met with Lucchino for 6 1/2 hours on Thursday and for four hours today. What the two sides agreed on was a compromise. The Orioles had offered McDonald a three-year, $735,000 contract, while the pitcher was seeking $1.1 million over three years. In the end they settled on something in between. The breakthrough came this morning when Boras offered to lower the 1990 salary request and load the contract with incentives. With that lower salary of $150,000, McDonald's pay will be closely in line with what other young players his age make. And that means the team's salary structure will remain lower. McDonald will make a base salary of $400,000 in 1991 and will get an additional $50,000 if he makes 30 starts. By raising the 1991 salary to $450,000, Boras also has assured McDonald his salary will be around $500,000 for 1992 -- the last year before he is eligible for arbitration. Two weeks ago the Orioles said they believed McDonald would join a proposed new baseball league that reportedly had offered him about $2 million. Today a source from that league said no formal offer was ever made, and sources close to McDonald acknowledged that was the case. They said their negotiating session in New York with representatives of the proposed league lasted about 15 minutes and that the answer was "Yes" to each of their proposals. They left the meeting believing the deal could be okayed with one phone call. When the Orioles heard of that offer, they held out only a slim hope of signing McDonald. The hope was that, despite the lure of big money, McDonald would cling to his dream of pitching in the major leagues. As it turned out that's what he did. Boras and Lucchino appeared to have broken off negotiations on Aug. 4, and they hadn't spoken again until Boras phoned this week. The Orioles are uncertain what the signing will mean in their clubhouse, although the price tag certainly will run into the millions. Reliever Gregg Olson, who was the fourth selection in the 1988 draft and will earn the major league-minimum $70,000 this year, received a $200,000 signing bonus. McDonald's signing will up Olson's asking price for 1990. The Orioles have the lowest payroll in the majors, with a $294,000 average salary. McDonald's $350,000 signing bonus makes him the fifth-highest-paid Oriole in 1989, behind only Cal Ripken ($2.4 million), Phil Bradley ($1.15 million), Larry Sheets ($660,000) and Dave Schmidt ($625,000). Although the contract is unique for a player out of college, scouts say the 6-foot-7 McDonald is unique. The Major League Scouting Bureau gave him its highest score in history, and the Orioles expect him to step into their rotation in 1990. "I don't know when I've ever rated a player so highly," Dodgers scouting director Ben Wade said. "You're talking about a guy that has everything you look for." McDonald's college coach, LSU's Skip Bertman, said, "I've seen Roger Clemens, Greg Swindell and Mike Moore at similar stages of their career, and Ben is ahead of all of them."